FIG. 3 is a conceptual diagram illustrating a configuration of electrodes of a cylindrical discharge tube according to the prior art, wherein 1 denotes a discharge tube, and 2a and 2b denote metal electrodes arranged along the outer periphery of the tubular wall of the discharge tube 1 and utilizing the tubular wall as a capacitive load. A high-frequency electric power of 2 MHz is supplied from a high-frequency, power supply 3, provided outside the discharge tube 1, through the electrodes to a laser gas flow 4 passing through the interior of the discharge tube 1 at a high speed. Numeral 5a schematically illustrates a transverse mode of the laser beam produced by this discharge pumping means. This transverse mode is a multimode, as explained hereinafter.
Optical elements and other elements necessary for a laser oscillator to produce a laser beam are omitted from FIG. 3.
FIG. 4 is a sectional view of the discharge tube in FIG. 3. As will be understood from FIG. 4, because the electrodes 2a and 2b are arranged on the outer periphery of the discharge tube 1, the closer they are to the tubular wall of the discharge tube away from the center thereof, the shorter the discharging distance between the opposed electrodes 2a and 2b, and thus the electric field is concentrated at the side edges of the electrodes 2a and 2b. Accordingly, a discharge current is liable to be concentrated at both ends of the discharge tube 1 and thus the current density per unit volume is large.
The distribution of the electric field strength is indicated by electric lines of force 6, and the laser active region is indicated at 7 in FIG. 4.
According to the prior art, the laser oscillation gain is lower at the center of the discharge tube than at the outer portions thereof, and thus a single-mode laser beam usable for a laser machining operation cannot be obtained. Accordingly, a multimode transverse mode of the laser beam is used, as indicated at 5a in FIG. 3.